The IUCC Transgenic and Knockout Mouse Core provides services for: (1) the production of transgenic mice (via pronuclear injection of recombinant DNA molecules) and (2) the production of knockout mice (via homologous recombination in ES cells). Core activities are as follows: (1) For transgenic mouse production, investigators provide the DNA construct to the core. The core then harvests single cell embryos, microinjects the embryos with the DNA construct, transplants the microinjected embryos into pseudo-pregnant females, performs Cesarean sections as required. At weaning age, the resulting pups are provided to the investigator for analyses; and (2) For knock-out mouse production, the core provides three services, namely ES cell transfection, blastocysts injections and rapid germ-line breeding. For ES cell transfection, the investigator provides the core with a targeting vector. The core then transfects ES cells with the targeting vector, selects the resulting transfected clones, generates frozen stocks of the clones, and provides the investigator with DNA samples from each of the clones for molecular analyses. For blastocysts injections, the investigator provides (or identifies) the desired ES clone. The core then recovers and amplifies the ES cell line, provides the investigator with cells for additional molecular and cell biologic analyses, generates back-up frozen stocks, injects the cells into blastocysts, transplants the injected blastocysts into pseudo-pregnant females, performs Cesarean sections as required, and culls the resulting chimeric animals. At weaning age, the chimeric pups are either provided to the investigator for analyses or are used for rapid germ-line breeding. For rapid germ line breeding, core personnel rotate female mice with the chimeric males. Plugged females are replaced, until a total of 10 pregnancies are culled. The resulting pups are screened for germ line transmission of the ES cell genome (determined via coat color). Tail biopsies are provided to the investigator for molecular confirmation of germ line transmission. At weaning age, the resulting heterozygous animals are provided to the investigator for analyses. Additionally, core personnel will provide advice concerning construction of transgenic and gene targeting constructs, animal breeding, and maintenance of the resulting mouse colonies. Overall, the core provides transgenic mice to IUCC investigators in a timely and cost-efficient manner.